"Muni" is a common abbreviation for "municipal", and sometimes becomes a frequently-used name for a city-related service or organization:

• Saint Louis Municipal Opera, America's largest outdoor musical theatre (spelled MUNY in this case)
• Municipal bonds
• San Francisco Municipal Railway, a city transit agency in San Francisco, California
• Springfield Municipal Opera
• Muni Consortium (broadband communications)
• Municipal Bridge (Saint Louis, Missouri)
• slang term for a municipally-owned and -operated golf course

A muni is a mountain unicycle.

A muni is a sage or monastic, primarily from Hinduism, where the term originated. It is also sometimes used in Jainism and Buddhism.

Paul Muni (1897-1967) was an American actor.

Scott Muni (1930-2004) was an American disc jockey.

Masaryk University Brno is often abbreviated to "Muni".

Rio Muni was a Spanish colony that is now part of Equatorial Guinea.